1. Field of the Invention
The subject invention generally relates to conductive polymers and more particularly to electrically conductive, static-dissipative, and anti-static polyoxymethylene compositions that have improved resistance to deterioration
2. Description of the Prior Art
Electrically conductive polymeric materials are desirable for many applications including the dissipation of electrostatic charge from parts, electrostatic spray painting, and the shielding of electrical components to prevent transmission of electromagnetic waves. The primary method of increasing the electrical conductivity of polymers is to fill the polymers with conductive additives such as metallic powders, metallic fibers, ionic conductive polymers, intrinsically conductive polymeric powder, e.g., polypyrrole, carbon fibers or carbon black. However, these approaches have shortcomings. For example, metallic fibers and polymeric powders have poor corrosion resistance and insufficient mechanical strength. Further, their density makes high weight loadings necessary; therefore, their use is frequently impractical.
When polyacrylonitrile (“PAN”) or pitch-based carbon fibers are added to a base resin to create conductive polymers, the high filler content necessary to achieve conductivity results in the deterioration of the characteristics specific to the base resin. If a final product with a complicated shape is formed by injection molding, uneven filler distribution and fiber orientation tends to occur due to the relatively large size of the fibers, which results in non-uniform electrical conductivity.
Carbon black has become the additive of choice for many applications. The use of carbon black, however, also has a number of significant drawbacks. First, the quantities of carbon black needed to achieve conductivity of the polymer are relatively high. Second, the high morphological “structure” of conductive carbon blacks is subject to breakdown during high shear melt processing. This morphological structure contributes to a reduction of toughness characteristics of a formed part to the point where the characteristics become too low for many applications. Even when toughness levels are suitable for a given application, the sloughing or rubbing off of the carbon black from the surface of the product may be a problem. Finally, the chemical impurities which are inherent in and result from the typical carbon black manufacturing process, make the use of these materials impractical in, for example, automobile parts.
Carbon nanotubes have been used in place of carbon black in a number of applications. For example, it has been recognized that the addition of carbon nanotubes to polymers in quantities less than that of carbon black, can be used to produce conductive end products. It has also been recognized that the addition of carbon nanotubes to polymers can be used to enhance the tensile and flexural characteristics of end products.
Carbon nanotubes are typically in the form of vermicular tubes with graphitic outer layers disposed substantially concentrically about the cylindrical axis of the nanotube. The nanotubes are typically substantially free of a pyrolytically deposited thermal carbon overcoat.
Carbon nanotubes typically have a length-to-diameter ratio of at least 5 and are carbon filaments that have diameters less than 500 nanometers. The wall thickness of the nanotubes is about 0.1 to 0.4 times the external diameter of the nanotubes, which is usually between 3.5 and 75 nanometers. In applications where high strength nanotubes are needed, e.g., where the nanotubes are used as reinforcements, the external diameter is substantially constant over its length.
One drawback to carbon nanotubes is that the carbon nanotubes are acidic and tend to escalate deterioration of the properties of the base resin. This deterioration tends to increase when the article formed from the composition is exposed to specific environments, such as high temperature or pressure environments or high fuel content environments. As one example, when the base resin is polyacetal, or polyoxymethylene, and when the components are used for automotive fuel systems, these components are subjected to a high fuel content environment. Therefore, the deterioration of the components is highly undesirable.